MD: warn for potential deadlock

The personality thread shouldn't call mddev_suspend(). Because
mddev_suspend() will for all IO finish, but IO is handled in personality
thread, so this could cause deadlock. To trigger this early, add a
warning if mddev_suspend() is called from personality thread.
Suggested-by: NeilBrown <neilb@suse.com>
Cc: Artur Paszkiewicz <artur.paszkiewicz@intel.com>
Signed-off-by: Shaohua Li <shli@fb.com>

drivers/md/md.c

@@ -305,6 +305,7 @@ static blk_qc_t md_make_request(struct request_queue *q, struct bio *bio)
  */
 void mddev_suspend(struct mddev *mddev)
 {
+	WARN_ON_ONCE(current == mddev->thread->tsk);
 	if (mddev->suspended++)
 		return;
 	synchronize_rcu();